1. Field of the Disclosure
The present disclosure relates to a resistive random access memory (RRAM) cell and RRAM module. More particularly, the present disclosure relates to a multilayer RRAM cell and a multilayer RRAM module.
2. Description of Related Art
RRAM has recently gained tremendous popularity due to its fast resistance switching while consuming low power. A key advantage of RRAM has been its ability to operate at lower operation current compared to other new nonvolatile memory candidates, such as phase-change memory (PCM). The operation current of RRAM is directly related to the cross-section area but to the input currents (ISET and IRESET) during the initial filament forming operation (SET operation) and the subsequent initial filament rupture operation (RESET operation). In order to reduce operation current of RRAM, the minimum resistance of the RRAM must be increased to an appropriate level.
FIG. 1 is a circuit diagram illustrating an oxide-based RRAM during SET operation, FIG. 2 is a circuit diagram illustrating an oxide-based RRAM during RESET operation. Referring to FIG. 1, the oxide-based RRAM 120 is electrically connected to a transistor 100. During the SET operation, a first voltage VGH,set is applied to turn on the transistor and a SET voltage VSET is applied between the anode 126 and cathode 122 resulting in the displacement of oxygen ions 160 in the oxide 124, leaving oxygen vacancies 150 behind, to generate a SET current ISET flowing through a percolating conduction path 170 formed of such vacancies (FIG. 1). During the RESET operation, a second voltage VGH,reset is applied to turn on the transistor and a RESET voltage VRESET is applied to generate a RESET current IRESET flowing through the transistor and the oxide-based RRAM sequentially (FIG. 2). Recombined vacancies 180 may be generated in the low current generated filament during the RESET operation.
FIG. 3 is a diagram illustrating the relationship between resistance of an oxide-based RRAM and ramping voltage. It is likely that a filament generated by a low current includes a broken path fairly separated oxygen vacancies, so that during the RESET operation, the oxide between traps may actually be broken down, causing a SET operation (i.e. filament re-generation) instead of RESET operation. As shown in FIG. 3, this inadvertent SET operation leads to a reduction of resistance of RRAM. Owing to the fact that the filament extends over a distance (e.g. 5-10 nm) less than the electron mean free path (>20 nm), the electron in the regenerated filament has ballistic transport, and therefore the filament is of low resistance (at least 1/G0=1/(2e2/h)˜13 k-ohm, where e is the electron charge, and h is Planck's constant.). During RESET operation, the input current (IRESET) must rise to value equal to the RESET voltage (VRESET) divided by resistance of this filament. For this example, for a RESET voltage (VRESET) of 1.3V, the RESET current (IRESET) rises to ˜100 μA. Ramping the voltage further, the actual RESET process begins, to be followed by progressive breakdown at an excessive voltage (above 2 V in this case). Due to this undesired effect, the reduction of operation current of RRAM below 100 μA may not be possible when RESET operation is performed, as the RESET voltage (VRESET) is continually ramped.
In a thicker film, the mean free path may be less than the thickness, and so there is no ballistic transport. However, thicker films have generally been avoided due to their much larger forming voltage (i.e. SET voltage, VSET).
It is possible to operate RRAM at extremely low currents (<μA) and consequently also very high SET state resistance. However, in this case, the SET operation may not produce well-defined filaments. As a result, simply operating RRAM by brute force ultra-low currents may not be satisfactory.